Internet Engineering Task Force SIP WG
Internet Draft G. Camarillo
Ericsson
J. Rosenberg
dynamicsoft
draft-ietf-mmusic-anat-00.txt
December 6, 2003
Expires: May 2004
The Alternative Network Address Types Semanticsfor the Session Description Protocol Grouping Framework
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Abstract
This document defines the Alternative Network Address Types (ANAT)
semantics for the SDP grouping framework. The ANAT semantics allow
offering alternative types of network addresses to establish a
particular media stream.
G. Camarillo et. al. [Page 1]

Internet Draft SIP December 6, 20031 Introduction
An SDP [1] session description contains the media parameters to be
used to establish a number of media streams. For a particular media
stream, an SDP session description contains, among other parameters,
the network addresses and the codec to be used to transfer media. SDP
allows providing a set of codecs per media stream, but only one
network address.
Being able to offer a set of network addresses to establish a media
stream is useful in environments with both IPv4-only hosts and IPv6-
only hosts, for instance.
This document defines the Alternative Network Address Types (ANAT)
semantics for the SDP grouping framework [2]. The ANAT semantics
allow expressing alternative network addresses (e.g., different IP
versions) for a particular media stream.
1.1 Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [3] and
indicate requirement levels for compliant SIP implementations.
2 ANAT Semantics
We define a new "semantics" attribute within the SDP grouping
framework [2]: ANAT (Alternative Network Address Types).
Media lines grouped using ANAT semantics provide alternative network
addresses of different types for a single logical media stream. The
entity creating a session description with an ANAT group MUST be
ready to receive (or send) media over any of the grouped m lines. The
ANAT semantics MUST NOT be used to group media streams whose network
addresses are of the same type.
3 Preference
The entity generating a session description may have an order of
preference for the alternative network address types offered. The
identifiers of the media streams MUST be listed in order of
preference in the group line. In the example below, the m= line with
mid=1 has a higher preference than the m line with mid=2.
a=group:ANAT 1 2
G. Camarillo et. al. [Page 3]

Internet Draft SIP December 6, 20034 Offer/Answer and ANAT
An answerer receiving a session description that uses the ANAT
semantics SHOULD use the address with highest priority it understands
and set the ports of the rest of the m= lines of the group to zero.
4.1 ANAT and Media Configurations
The creator of a session description MAY want to use different media
configurations (e.g., audio codec) for different network addresses in
the same ANAT group. The receiver of such a session may find some of
the m lines unacceptable. They may contain codecs that the answerer
does not support or contain any other parameter that makes them
unacceptable. The answerer should, following normal SIP procedures,
set their ports to zero in the answer.
5 SIP Option-Tag
We define the option-tag "sdp-anat" for use in the Require and
Supported SIP [4] header fields. SIP user agents that place this
option-tag in a Supported header field understand the ANAT semantics
as defined in this document.
Using the sdp-anat option-tag in a Require header field allows a user
agent to explicitly discover whether or not the remote end supports
the ANAT semantics. Nevertheless, user agents MAY use the ANAT
semantics without using the sdp-anat option tag. In this case, an
offer with an ANAT group may be received by a user agent without
support for it. Such a user agent may refuse the offer because it
contains unknown address types or may only establish the media
streams whose address types understands (it would reject the rest.)
If this behavior is not acceptable for the generator of an offer, it
MUST use the sdp-anat option-tag in a Require header field.
6 Example
The session description below contains an IPv4 address and an IPv6
address grouped using ANAT.
v=0
o=bob 280744730 28977631 IN IP4 host.example.com
s=
t=0 0
a=group:ANAT 1 2
m=audio 6886 RTP/AVP 0
c=IN IP6 2001:0600::1
G. Camarillo et. al. [Page 4]

Internet Draft SIP December 6, 2003
a=mid:1
m=audio 22334 RTP/AVP 0
c=IN IP4 192.0.2.2
a=mid:2
7 IANA Considerations
IANA needs to register the following new "semantics" attribute for
the SDP grouping framework [2]:
Semantics Token Reference
----------------------- ----- ---------
Alternative Network Address Types ANAT [RFCxxxx]
It should be registered in the SDP parameters registry
(http://www.iana.org/assignments/sdp-parameters) under Semantics for
the "group" SDP Attribute.
This document defines a SIP option-tag (sdp-anat) in Section 5. It
should be registered in the SIP parameter registry
(http://www.iana.org/assignments/sip-parameters.)
SIP user agents that place the sdp-anat option-tag in a
Supported header field understand the ANAT semantics.
8 Security Considerations
An attacker adding group lines using the ANAT semantics to an SDP
session description could make an end-point use only one out of all
the streams offered by the remote end, when the intention of the
remote-end might have been to establish all the streams.
An attacker removing group lines using ANAT semantics could make and
end-point establish a higher number of media streams. If the end-
point sends media over all of them, the session bandwidth may
increase dramatically.
It is thus STRONGLY RECOMMENDED that integrity protection be applied
to the SDP session descriptions. For session descriptions carried in
SIP [4], S/MIME is the natural choice to provide such end-to-end
integrity protection, as described in RFC 3261. Other applications
MAY use a different form of integrity protection.
9 Authors' AddressesG. Camarillo et. al. [Page 5]

Internet Draft SIP December 6, 2003
Gonzalo Camarillo
Ericsson
Advanced Signalling Research Lab.
FIN-02420 Jorvas
Finland
electronic mail: Gonzalo.Camarillo@ericsson.com
Jonathan Rosenberg
dynamicsoft
72 Eagle Rock Ave
East Hanover, NJ 07936
USA
electronic mail: jdrosen@dynamicsoft.com
10 Normative References
[1] M. Handley and V. Jacobson, "SDP: session description protocol,"
RFC 2327, Internet Engineering Task Force, Apr. 1998.
[2] G. Camarillo, G. Eriksson, J. Holler, and H. Schulzrinne,
"Grouping of media lines in the session description protocol (SDP),"
RFC 3388, Internet Engineering Task Force, Dec. 2002.
[3] S. Bradner, "Key words for use in RFCs to indicate requirement
levels," RFC 2119, Internet Engineering Task Force, Mar. 1997.
[4] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. R. Johnston, J.
Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP: session
initiation protocol," RFC 3261, Internet Engineering Task Force, June
2002.
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G. Camarillo et. al. [Page 6]

Internet Draft SIP December 6, 2003
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